July 02, 2013
Matt O. and Michael K. asked related questions: If a red dwarf's solar flares grew less intense as the star got older, might there be a window in between the period where the intensity of the solar flares would preclude life and the star dying that would allow the development of advanced life? Why is Gliese 581e not considered an Earth-type planet when it is less than twice as big as Earth? Its orbit is close like Mercury but it revolves around a red dwarf so maybe that's just the right amount of heat.
Matt’s and Michael’s questions point out both the possibilities and the problems of life starting and then getting established on planets orbiting red dwarf stars.
Because red dwarfs are cool and small compared to the Sun, their habitable zones (wherein water would remain a liquid on a planetary surface) are much closer to the star. That’s not a problem, but other characteristics of the stars and the laws of physics complicate the picture.
Plumes of plasma
It is common for red dwarfs to have stellar flares like the Sun’s solar flares, often more powerful than the Sun’s flares. These are explosive releases of energy above a star’s “surface” (it’s all gas, so there is no true surface) caused by the reconnection of magnetic field lines. The reconnection generates ultraviolet and x-rays (and even gamma rays, on the Sun) and can accelerate large amounts of plasma (electrically charged gas) outward from the star. The UV and x-rays can heat a planet’s upper atmosphere and the flowing plasma can sweep it away from the planet. It is believed that enough flares could strip a planet of its atmosphere.
Because even solid planets are slightly flexible, tides on a planet caused by the gravity of its host star will eventually slow its rotation. Over time the planet will rotate at a rate that just matches the time for it to go around the star (its year). Our Moon does the same thing, keeping the same face toward Earth, making one rotation and one revolution every 29.5 days as seen from the ground. Synchronous rotation like this will have interesting effects on a planet’s climate – hot, perpetual sunlight on one hemisphere and cold, perpetual night on the other hemisphere.
Red dwarfs: Is anyone home?
Could life evolve on a habitable zone planet orbiting a red dwarf? Sure. If the star is unusually quiet or if an atmosphere is generated when the star quiets down (and hasn’t cooled too much) there’s a chance for life to take hold. Even with a planet in synchronous rotation, the “twilight zone” between the hot and cold hemispheres might be suitable for life.
A quick, back-of-the-envelope calculation suggests that Gliese 581 is about 100x less luminous than the Sun. That means that the equivalent of Earth’s comfortable distance from the Sun would be 10x closer to Gliese 581, at a distance of 0.100 astronomical unit (AU). Gliese 581e is 0.028 AU from its host star, about ¼ of Earth’s equivalent distance and less than 1/10th of Mercury’s distance (0.31-0.47 AU). It is well inside (and too hot) for it to be Earth-temperature even if the planet were spinning and not in synchronous rotation. There could be life there but it probably doesn’t resemble anything we are familiar with.